1. Field of the Invention
This invention relates to a so-called magnet pump in which an impeller having a permanent magnet embedded therein is rotatably driven by a magnetic force from the outside and, particularly, to a magnet pump adopting an axial gap type magnetic force transmitting mechanism in which a magnet embedded in an impeller is rotatably driven by a magnetic force from a permanent magnet or stator provided in a position spaced therefrom in the direction of the rotary shaft axis of the impeller, and in which the liquid-contacting portion has a simple structure which is suitable for pumping and circulating various chemicals.
2. Prior Art
Typical examples of such a magnet pump are shown in FIGS. 6 and 7. The magnet pump shown in FIG. 6 is adapted to rotatably driven the pump impeller utilizing the magnet coupling between permanent magnets and the pump comprises a pump casing 1 including an inlet cover 1a joined to a back cover 1b and including a pump chamber 1c containing the impeller 2, a suction port 1d for introducing fluid to be pumped and a discharge port 1e for discharging the pressurized fluid. The back cover 1b is provided on its non-liquid-contacting portion with a magnet yoke 3a fixed to a main shaft 3 rotatably driven by a drive source, such as a motor (not shown), and a permanent magnet 3b is annularly provided on a surface of the magnet yoke 3a opposing the back cover 1b. An annular permanent magnet 2c is embedded in a similarly annular magnet yoke 2b within the impeller 2 which is rotatably contained in the pumping chamber 1c, and the impeller 2 is rotatably driven by the main shaft 3 to displace the fluid. The annular permanent magnet 2c and permanent magnet 3b may be formed in a single ring-like permanent magnet or of a plurality of annularly arranged permanent magnets. In any case, on the mutually opposed surfaces of the respective permanent magnets 2c and 3b, the N- and S-poles should be alternately and circumferentially arranged. The force acting on the impeller 2 when it is not moved is mainly the magnetic attraction force F1 of the permanent magnets 2c, 3b and thus the impeller 2 is urged towards the back cover 1b and the pump is activated in such a condition. Therefore, the back cover 1b is provided on its side with a fixing-part bearing 1f and the impeller 2 also on its back surface with a rotating-part bearing 1g to thereby support the thrust and radial loads.
With respect to the bearing for supporting the impeller for rotation, a slide member on the rotating part (bearing 1g) and a slide member on the fixed part (bearing 1f) are each referred to as a bearing and a pair of such slide members also to simply be referred to as a bearing herein.
In FIG. 6, the arrow indicating the magnetic attraction force F1 acting on the impeller 2 does not exactly indicate where the force acts upon, but merely shows a component of the magnetic attraction force in the direction of the rotary shaft axis. As the impeller 2 rotates, the fluid is pressurized and the fluid pressure acts as a thrust force F2 for urging the impeller 2 towards the inlet side. Thus, the inlet cover 1a is provided with a fixed-part bearing 1h, and a rotating-part bearing 2j is provided on a part opposing the bearing 1h of the impeller 2. The strength of the magnetic attraction force F1 generated by the permanent magnets 2c, 3b varies due to the fluid force applied to the impeller 2, and the magnitude of the thrust force F2 also varies due to the fluid pressure. A case in which the pump impeller is rotating at a constant speed will now be explained with reference to the pressure-flow rate characteristic curve shown in FIG. 6(a) of which the ordinate H is the pump discharge pressure and the abscissa Q is the pump flow rate. Since the centrifugal pump as shown in FIG. 6 normally starts with the outlet valve closed off, the pump operating point is the point A, and when the outlet valve is gradually open, the pump operating point moves along the solid line a to the point B. Between the points A and B, the pump impeller 2 rotates, while it is subjected to a thrust force F2 which is larger than the magnetic attraction force F1, and urged towards the inlet cover 1a. At the point B, the magnetic attraction force F1 and the thrust force F2 are equal to each other, and as the valve is further opened the magnetic attraction force F1 becomes larger than the thrust force F2 and the impeller 2 rotates while urged towards the back cover 1b (point C). At point C, the impeller 2 is rotating at a position spaced away from the inlet cover 1a and therefore the bearing clearance between the high and lower pressure regions becomes sufficiently large to allow the pressurized fluid to escape to the suction portion thereby reducing the pump outlet pressure below the point B. When the valve is opened still further, the pump operating point moves along the solid line to the point D. When the valve then gradually closes the pump operating point moves to the point E through the point C since the magnetic attraction force F1 is larger than the thrust force F2. At point E, the magnetic attraction force F1 and the thrust force F2 are equal to each other. When the valve is opened further the impeller 2 rotates while being urged by the thrust force F2 towards the inlet cover 1a and the operating point reaches the point F. In this manner, according to the magnet pump of FIG. 6, the pump pressure-flow rate characteristics curve depicts a hysteresis curve, and accordingly the impeller 2 rotates while biased either towards the inlet cover 1a or towards the back cover 1b depending upon the operating condition, as described above.
A further conventional magnet pump shown in FIG. 7 is adapted to directly rotatably drive the magnet embedded in the impeller with the electromagnetic force generated by the stator. The basic structure of the casing 1 and impeller 2 is the same as those of the conventional pump shown in FIG. 6, but it differs in that the driving magnetic force mechanism constituting the means for rotatably driving the impeller 2 is a stator 5. The stator 5 for driving the magnet embedded in the impeller is mounted on the non-liquid-contacting portion of the back cover 1b at a position opposing the magnet of the impeller, the stator 5 having coils 5a wound on an annularly arranged core 5b, the coils 5a being supplied with power from a power source control circuit not shown, thereby rotatably driving the impeller 2 on the operating principle of a so-called brushless motor. The magnet pump of FIG. 7 also exhibits similar behavior on the characteristic curve to those of the pump of FIG. 6 described above, and the impeller 2 rotates while based either towards the inlet cover 1a or towards the back cover 1b depending upon the pump operating condition.
In the aforementioned magnet pumps, the impeller 2 is, in operation, shifted in the direction of the rotary shaft axis 20 thereof depending upon the operating condition. Namely, in respect to the axial force acting on the impeller 2, there are operating regions, i.e., an operating region (A.fwdarw.B, F.fwdarw.A on the characteristic curve) in which the resultant force of the magnetic attraction force F1 acting between the permanent magnet 2c embedded in the impeller 2 and the driving-part permanent magnet 3b (or stator 5) for rotatably driving it and the thrust force F2 due to the fluid acting on the impeller is directed towards the inlet cover 1a, an operating region (C.fwdarw.D, D.fwdarw.E) in which the resultant force is reversely directed towards the back cover 1b, and an unstable region (B.fwdarw.C, E.fwdarw.F) which is the transient region between the operating regions, and the position of the impeller may vary depending upon the pump operating conditions in various ways. Upon such an axial shift of the impeller 2, an impact load is applied to the bearing and, since the impeller is then rotated with eccentricity at a corresponding speed, uneven contact occurs on the sliding surface of the bearing which causes damage. Such a phenomenon creates problems, particularly when air bubbles are contained in the fluid to be pumped. Namely, when fluid containing air bubbles is pressurized under the action of the centrifugal force of the impeller 2, the discharge pressure widely varies and accordingly the thrust force F2 caused by the fluid pressure also varies. Thus, the impeller 2 is vibrated in the axial direction, which can cause undesirable pump vibration and the possibility of bearing failure. In the aforementioned prior art examples, moreover, bearings are provided in the respective positions on the sides of the inlet cover 1a and back cover 1b and these bearings must be assembled parallel or perpendicular to each other and this causes problems in the machining and assembling processes of the parts.
In the case that extremely high purity fluid is to be delivered without being contaminated, an easily damageable bearing involves the risk of bearing debris entering into such a fluid, and if a special surface treatment such as a corrosion resistance is applied to the bearing, the layer of surface treatment may be peeled off; these represent potentially serious problems which are likely to occur even if the pump is well maintained.